Technical Abstract: In 2006, EPA implemented a more stringent standard for PM2.5, particulate matter whose effective diameter is less than 2.5 microns. PM2.5 is listed as a criteria pollutant in the National Ambient Air Quality Standards (NAAQS). Agricultural operations across the United States are encountering difficulties in complying with the current air pollution regulations due to limited PM2.5 data. Research efforts focused on closing this data gap are currently underway. Most agricultural point sources are regulated based on results obtained from dispersion modeling that utilize emission factors from EPA’s AP-42 or emission factors derived from source sampling. Modeling of PM2.5 emissions from some sources is difficult or impossible due to the current lack of PM2.5 data, especially in EPA AP-42. PM2.5 emission factors are typically determined from source sampling based on EPA’s Method 201a sampling protocol. Method 201a utilizes a pair of cyclones in the sampling system to remove the larger particles and allow the smaller particles to penetrate to the filter. EPA has documented the performance characteristics of the cyclones used in Method 201a, which are similar to the performance characteristics of an ambient PM2.5 sampler. Recent research has shown that ambient PM2.5 samplers can over-estimate the true PM10 in the ambient air when the sampler is exposed to dust with a mass median diameter larger than 2.5 'm. The purpose of this manuscript is to explore the potential impacts associated with determining PM10 emission values for using the EPA’s Method 201a. These tests were conducted in a laboratory setting using test materials with known and consistent particle size characteristics.